Traveling-wave tube



APl'llfsl 1958 v l s. E; wEBBER l 2,830,219

v -TRAvELING-WAVE TUBE.'

Filed June 29, 195o Figi.

Inventor: Stanleg B'Webber,

bg His Attorneg.

TRAVELlNG-WAVE Stanley E. Webber, Ballston Lake, N. Y., assigner toGeneral Electric Company, a corporation of New York l Application June29, 1950, Serial No. 171,178

Claims. (Cl. S15-6.5)

This invention relates to improvements in'electron dis-Y charge devicesof the type generally called traveling-wave tubes; and its principalobject is to increase the maximum power output, the gain, and theefficiency Vor such tubes, especially for use aspower ampliers. Otherobjects and advantages will appearzas the description proceeds.

The features of this invention which are believed to be novel andpatentable are pointed out in the claims which form a part of thisspecification. For a better understanding of the invention, reference ismade in the following description to the accompanying drawing, in whichFig. l is a schematic longitudinal section of a travelingwave tubeembodying principles of this invention; Fig. 2-

is a fragmentary side view of the same tube showing details of theattenuator construction; and Fig. 3 is a fragmentary longitudinalsection of a modiiication of the improved traveling-wave tube.-

Referring now to Fig. 1 of the drawing, the traveling-f wave tubeamplies an electromagnetic Wave transmitted between input terminal 1 andoutput terminal 2. Connecting these terminals is a kstructure whichtransmits electromagnetic waves at relatively low velocity. Thisstructure may be an electrically conductive wire helix 3, as shown inthe drawing. Since an electromagnetic wave travels along the wire ofhelix 3 at substantially the speed of light, the velocity of the wavealong the axial direction of the helix isless than the speed of lightbyafactor substantially proportionalto the ratiobetween the pitch ofhelix 3 and its diameter. It is known in the, art Athat other structureswhichA transmit electromagnetic waves at relatively low velocity maybefused in place of the helix, and it will be understood that such otherstructures may be used in the practice of my invention. Examplesofstructures which have beenY proposed for such use arel a rod which iselectrically loadedbwith a plurality of rather closely spaced transversemetal kdisks,. or a cylindrical wave guideloaded with a' pluralityoflapertured disks.

United States Patent C theeld of the transmitted electromagnetic wave isConnection of terminals 1 and 2 to external circuits may be made bydirect connections to the wire, or by inductive orV capacitativecoupling, e. g., by a probe, cavitylor wave-guide connection, as isknown in the art.

Closely surrounding and enclosing helix 31is a tubular envelope 4 whichis usually made of electrically 'noncon-Y This envelope is evacu ductivematerial, such as glass. ated in the customary manner. An alternativestructure is one in ywhich the evacuated envelope is a metal cylinder atsome distance outside thehelix, within which the helix is supported byinsulatingrods.

Within theleft-hand end of envelope 4V there is an electron guncomprising a cathode 5, a cathode heater 6, and a focusing electrode 7.Electrode 7 may be tubular in shape, so that a beam of electrons,indicated by dotted lines 8, is directed axially through helix V3 to acollector plate 9 positioned `within the right-hand end of envelope 4.Means, n ot shown, may be provided to produce a longitudinallmagneticeldthrough the envelope to keep strongest. A battery 10, or othersuitable voltage source, is connected to maintain electrode 7 andcollector plate 9 at a large positive potential with respect to cathode5, and a batttery'll, or other current source, is provided to supplyoperating current to heater 6. Y v Y.

The ratio of pitch to diameter of helix 3 and the electron acceleratingpotential provided by battery 10 areso related that electron beam 8travels in the same direction and at near the same velocity aselectromagnetic waves transmitted by-helix 3 between the input terminal1 and output terminal 2. lThe adjustment is such that the veloc ity ofthe electron beam is slightly greater than'that of the wave type withwhich it is to react. Under these conditions, it is known thatinteraction between the beam and the wave occurs in such a way that theelectromagnetic wave is amplified as it travels along the helix. One exfplanation of this interaction, by l. R. Pierce, may be found in The BellSystem Technical Journal, vol. 29,` No. 1

(January 1950),- pp. 6-19.

Because the traveling-wave tube is capable of amplifying a very broadband of frequencies, it is ditiicult to match exactly the impedances ofthe connecting circuits Y at input terminal 1 and output terminal 2 tothe impedance of the helix at all frequencies which the tubecan amplify.Therefore, at certain frequencies there may be reliectio-ns whereby anamplied wave is rellected from the output back to the input, where it isreilected again and further amplified as it travels back toward theoutput. This maylproduce oscillations unless an attenuator is 'providedYsuch that the attenuation of the wavein the back# ward direction exceedsthe gain in the forward direction. Several ways of providing thisattenuation have been suggested, such as makingthe helix itself of alossy material.

However, the most convenient way of providing the neces' saryattenuation is to provide an attenuator 12 of lossy,

electrically conductive material about the outside, or the' inside or asa part, of envelope 4. This attenuator may,

for example, be a platinum band positioned about the out side of theenvelope, or an Aquadag coating 'applied to the surface of the envelope.

the envelope, an attenuator so placed absorbs energy from the field,'andcan easily be made to provide the requiredA attenuation. Some of theattenuators previously used ex-v tend substantially throughout theentire length of the helix.rv However, it has been found that arelatively short attenuator can be employed to greater advantage.

According to known prior art, the location of the atten-- uator alongthe helix was believed to be unimportant.

When a short attenuator was employed, it was usually placed near thecenter of the helix. According to the presfent invention, however, thelocation of the attenuator has an important eifect upon the maximumpower output,

gain, and etliciency of the tube, as will now be explained. Best resultscan be obtained by placing attenuator 12k near the input terminal of thehelix, as shown in Fig. l, rather than near the center of the helix asin previous practice. It has been found that maximum power output fromthe tube can be obtained when attenuator 12 is placed as close aspossible to input terminal 1, and that` Y the maximum output powerdecreases as the attenuator Patented Apr. 8, 1958 Since a portion of thefield of the transmitted electromagnetic Waveextends through helix.Therefore, for a given percentage attenuation of the' wave, less powerisl absorbed when the attenuator is nal, as' shown inA Fig. l. However,since the attenuatorl is still relatively close to the inputterminal,the maximum poweroutput obtainable' is' not" far below the optimumvalue. Therefore, by'placing the attenuator for maximum gain,themaximum'powerj output is alsoA increased over that obtainable with'previousarrangements,"and is veryV near the optimum value. The eiciencyVof the tube is also increased, since more; power output is obtainedwithout increasinguthe input power supplied to the electronbeam. This isan especially important advantage when the traveling-wave tube is to beused asY a power amplifier, since, previously, aserious` objection totraveling-wave tubes for such use has been-their relatively low eciency.

T heioptimum spacing for maximum gain between input terminal 1 andattenuator 12 isinthe order of one wavelength of `the electromagneticwave as transmitted by helix 3. Since the traveling-wave tube is anextremely broad-band device, this distance, expressed in wavelengths,may vary considerably, even for the-same tube when Ytransmittingdifferent wavelengths. A spacing as small as. one-half wavelength, or aslarge as two wavelengths of `the transmitted wave may giveV goodresults. However, in all cases the position of the attenuator isrelatively close to input terminal 1, since the total length of thehelix` in an efficientA traveling-wave tube may be seven or morewavelengths of the electromagnetic wave n as transmitted by the helix.

The attenuator itself should be as short as it' can be made while stillproviding. a sutcient amount of attenuation to prevent oscillations. Inapractical tube, the spacingbetween inputv terminal 1 andthe attenuatormay be in theV order of 20%, or less, of the helix length, while thespacing between the attenuator and output terminal 2 may be in the orderof 60%, or more, of the helix length. Thus, the latter spacing ismuch.greater than the former.

A possible` explanation of theincreascdgain obtained by placing theattenuator a short. distance fromV the input terminal follows: In thespace between the inputv terminal' andthe attenuator, the electron beam.is velocityfmodulated tosome extent. by the electromagnetic wavehansmitted by the helix. This. is` especialy true when the signal isVrelatively large, as in power amplifier applications oi the tube. Thatportion of the helix adjacentthe attenuator. then actsas a drift spacein which'electron bunching takes place. The hunched electrons induce anelec-` tromagnetic wave Vin'thatportion of the helix between theattenuator and output 'terminal` 2, thereby providing considerableamplification of` the transmitted wave. If the attenuator is placed asclose as possible to input terminal 1, the immediate attenuation of theinput signalV reduces this'. velocity modulation of thef beamV by anamount which depends uponthe rate of attenuation,` and therefore thegain` obtained from bunching'of the electrons is reduced. The wave can'be attenuated soV fast that there is no` velocity modulation ofthebeam. On the other hand, the necessary `"velocity modulation occurswithin a relatively short distance, in the order of one wavelength ofthe transmitted wave, and any additional spacing` between the `inputterminal and the attenuator produces no useful result, but actuallyreduces the gain, as well as the maximum output power and eiiiciency.`

To prevent successive reflections between. the oufpu terminal and theright-hand edge of the attenuator, which could produce undesiredoscillations, 1h@ fight-hand edge of the attenuator is preferablyserrated, as shown in Fig.

2, so that there is no abrupt impedance discontinuity to createreflections at the right-hand edge of the attenuator.

Traveling-wave tubes embodying this invention may be used either asamplifiers or as oscillators, depending upon the external circuitemployed with the tube. When used in an oscillator, a positive feedbackcircuit is provided between the output and input terminals. Since thetube does not oscillate in itselhthe frequency of oscillation can easilyand accurately be controlled by the feedback circuit.

It should be understood that Vvthe theoretical explanations given inthis specicaticn are for clarity only, `and that applicants invention`is not limited to any particular theory of operation of thetraveling-wave tube.

Having described the Vprinciple of this invention, and the best mode inwhich I'have contemplated applying that principle, I wish it to beunderstood that the apparatus described isv illustrative'only, and thatother means can be employed without departing from the true scope of theinvention.

What 1 claim asl new and desire to secure by Letters Patent of theUnited States is:

l. An electron discharge device comprising a transmission structurehaving input and output terminals between which electromagnetic wavesare transmitted at low velocity, means providing an electron beam inproximity to said structure traveling in the same direction and at nearthe same velocity as an electromagnetic wave so transmitted, wherebyinteraction between said beam and said wave occurs'to amplify the wave,and attenuator means placed in proximity to said structure within theeld of electromagnetic waves-transmitted thereby having a lengthsubstantially less than half that of said transmission structure,characterized in that said attenuator is spaced from the input terminalof said structure a distance not exceeding two wavelengths of theelectromagnetic wave as transymitted by said structure and is spacedfrom the output terminal of said structure by a much greater distance,said attenuator means beingthe only attenuating means in proximity tosaid transmission structure between the nput and output terminalsvthereof so that substantially more attenuation is present inthe inputhalf. of said transmission structure than in the output halfV of saidtransmission structure. i v

2. Anelectron discharge device comprising a transmission structurehaving input and output terminals between which electromagnetic waves`are transmitted at low velocity, means providing an electron beam inproximity to said structure traveling in the same directionV and atnear'the-samevelocity as-an electromagnetic wave so transmitted, wherebyinteraction between said beam VVand said waveV occurs to'amplify thevwave, and attenuator means placed in proximity to said Ystructure withinVthetield ofelectromagnetic waves transmitted thereby,

the lengthY `of vsaid attenuator being substantially less than half'thelength of said structurecharacterized in that said attenuator is spaced`from the input terminal of said structure a Vdistance not exceedingone-fifth the total length of said'` structure-and from. said outputterminal a distance exceeding 3A; the total length ofsaid transmissionstructure, said attenuator means being, the only `attenuating means inproximity to said transmission structure between the input and outputterminals thereof so that substantially moreattenuation is present inthe input half of said transmission structure than in the output half ofsaid transmission structure. i

3. An electron discharge device comprising a transmissionstructurehaving input. and output terminals between which electromagnetic wavesare. transmitted at low velocity, means providing an electron beam inproximity to said structuretraveling in the same direction and at nearthe same velocity as an electromagnetic wave so transmitted, wherebyintersection between said beam Aand' said wave o ccurs toamplify thewave, and

attenuator means placed in proximity to said structure Within the fieldof electromagnetic Waves transmitted thereby, the length of saidattenuator being short compared to the length of said structure,characterized in that said attenuator is spaced from the input terminalof said structure a distancein the order of one wavelength of theelectromagnetic wave as transmitted by said structure, said attenuatoralsoV having a gradually decreasing volume toward the edge thereoflnearer said output terminal to eliminate abrupt impedancediscontinuity, said attenuator means being the only' attenuating meansin proximity to said transmission structure between the input and outputterminals thereof so that substantially more attenuation is present inthe input half of said transmission structure than in the output half ofsaid transmission structure.

4. A traveling-wave tube comprising input and output terminals, a long,electrically conductive helix connected between said terminals fortransmitting electromagnetic waves therebetween at low velocity, anelectrically nonconductive, tubular, evacuated envelope closelysurrounding and enclosing said helix, means providing an electron beamaxially through said helix, said last-named means comprising an electrongun within one end of said tubular envelope and a collector plate withinthe other end of said envelope, a cylindrical attenuator of lossy,electrically conductive material about said envelope and Within the eldof electromagnetic waves transmitted by said helix, said attenuatorbeing short compared to the length of said helix, said attenuator beingspaced from said input terminal a distance not exceeding one-iifth thelength of said helix, the edge of said attenuator nearest said outputterminal being serrated to prevent the occurrence of an abrupt impedancediscontinuity which might produce reflections, said cylindricalattenuator being the only attenuator associated` with said helix solthat substantially more attenuation is present in the half of saidhelix adjacent said input terminal than is present in the half of saidhelix adjacent said output terminal. 5. A11 electron discharge devicecomprising a transmission structure having input and output terminalsbetween which electromagnetic waves are transmitted at low velocity,means providing an electron beam in proximity to said structuretraveling in the same direction and at near the same velocity as anelectromagnetic wave so transmitted, whereby interaction between saidbeam and said wave occurs to amplify the wave, and a single cylindricalattenuator placed in proximity to and surrounding said structure withinthe eld of electromagnetic Waves transmitted thereby, the length of saidattenuator being short compared to the length of said structure,characterized in that said attenuator is placed immediately adjacentsaid input terminal said cylindrical attenuator being the onlyattenuator associated with said transmission structure so thatsubstantially more attenuation is present in the half ofthe transmissionstructure adjacent said input terminal than is present in the half ofsaid transmission structure adjacent said output terminal.

References Cited in the file of this patent UNITED STATES PATENTS2,122,538 Potter July 5, 1938 2,278,210 Morton Mar. 31, 1942 2,516,944Barnett Aug. l, 1950 2,585,582 Pierce Feb. 12, 1952 2,660,689 Tourstonet al Nov. 24, 1953 2,669,674 Diemer Feb. 16, 1954

